To arrive at Connecticut AGI, should federal AGI be increased by tax exempt interest and reduced by premium on tax exempt bonds? A prior post says bond premium did reduce Tax Exempt Interest as it does on the Federal 1040. But not happening on CT 1040

[JohnW152 · Accepted solution]

Interest on state and local government obligations from jurisdictions other than Connecticut, not taxed for federal income tax purposes, is added into federal AGI to arrive at Connecticut AGI.  See Line 31: Interest on State and Local Government Obligations Other Than Connecticut in the Form CT-1040 Connecticut Resident Income Tax Return Instructions. 

As for premiums on tax-exempt bonds, according to section 5 of Line 37: Other in the instructions:

Add back any amortizable bond premium on bonds producing interest income exempt from Connecticut income tax which premiums were deducted on the federal return to arrive at federal adjusted gross income.

And, according to section 6 in Line 49: Other, you can:

Subtract any amortizable bond premium on bonds that provide interest income taxable in Connecticut but exempt from federal income tax, which premiums were not deductible in determining federal adjusted gross income and are attributable to a trade or business of that individual.

[Bolding mine.]

Additions and subtractions to federal AGI are calculated on Schedule 1 - Modifications to Federal Adjusted Gross Income.  See Form CT-1040.